1. Field of the Invention
The present invention relates to a method of turning the surface of a die, which is used to make a part to put into the lens barrel of a camera, for example, into an antireflective roughened one.
2. Description of the Related Art
A camera such as a still camera or a camcorder receives external light, produces a subject image using an optical system inside its lens barrel, and then projects the subject image onto an image capturing member such as a photosensitive film or an image capture device. In this case, only the subject image that has been produced by the optical system is preferably incident on the image capturing member. Actually, however, there is also other light that has been reflected from inside the lens barrel before impinging on the image capturing member. Such unnecessary reflected light will be superposed as “flare” on the subject image. It is preferred that no flare be produced unless it is used as a sort of special video effect.
To eliminate such flare as much as possible, a part with an antireflective roughened surface is usually arranged inside the lens barrel. As used herein, the “roughened surface” refers to an object surface with a leather pattern, a sand pattern, a sprinkled lacquer pattern, a geometric pattern or any other form of regular or irregular roughness.
Inside the lens barrel, there may be a resin molded part, which often has such a roughened surface. And the surface of a die to make such a part is roughened in advance. Hereinafter, a conventional method for roughening the surface of a die will be described with reference to FIGS. 7 through 9.
FIG. 7 illustrates the shape of an abrasive grain 5 for use in the conventional surface roughening method. The abrasive grain 5 is used in sandblasting and has a size d, which may be approximately 0.01 mm to 2 mm. The abrasive grain 5 is made of aluminum oxide, for example.
FIG. 8 is an enlarged cross-sectional view illustrating the surface of a die 6 and its surrounding portions when blasting is carried out by the conventional surface roughening method.
When abrasive grains 5 are repelled by compressed air, for example, to collide against the surface of the die 6, mountains 6a and valleys 6b are formed.
FIG. 9 is an enlarged cross-sectional view illustrating the surface of a resin molded product 7 that has been molded using the die 6 and its surrounding portions. The valleys 7a of the molded product 7 have been formed by transferring the mountains 6a of the die 6, while the mountains 7b of the molded product 7 have been formed by transferring the valleys 6b of the die 6. And these valleys 7a and mountains 7b form the roughened surface of the molded product 7.
Japanese Utility Model Application Laid-Open Publication No. 2-60045 discloses a technique for forming a finely roughened surface around the periphery of a rearview mirror for cars by making superfine particles collide against the periphery by known blasting process. As a result, it says, in a rearview mirror for cars, unwanted reflection of the light that has come from the periphery and impinged on the mirror surface will be minimized.
According to the conventional method of roughing the surface shown in FIGS. 7 to 9, the abrasive grains 5 easily collide against the mountains 6a of the die 6, thus forming a complex shape on the surface as shown in FIG. 8. On the other hand, the abrasive grains 5 do not collide against the valleys 6b easily and will rarely form a complex shape there. If a resin molding process is carried out using such a die, the mountains 7b of the molded product 7 will reflect the relatively smooth shape of the valleys 6b of the die 6, and therefore, will have a relatively smooth surface, too. In that case, the mountains 7b of the molded product that are located at the uppermost surface thereof will reflect the incoming light easily.
That is why even if the resin molded part inside the barrel is shaped by such a conventional surface roughening method, the incoming light will still be reflected significantly. Thus, when such a resin molded product is used as a part inside the lens barrel, flare will be produced. Also, if a non-reflective surface is required from optical design considerations, the surface of a molded product is sprayed with a special antireflective paint. However, it is expensive and interferes with realizing an inexpensive product to spray the surface with such a special paint.